"JohnC" wrote in message
.. .
My Harmon Kardon receiver recently started to hum. It does it regardless
of which input is selected. The hum varies with the volume control. I
opened the unit and tapped on various components with a wood stick. This
had no affect on the hum.

This is an old unit that I have owned new since early 70s. Perhaps this
might not be worth getting serviced by a shop? I'm good with tools but
lack electronics skills. No scope either. What could be the cause of the
hum? Is this something that is fairly easy to replace/repair?

Here is a pic of the circuit board:

http://home.everestkc.net/upperquad/...rmonKardon.jpg

Thanks
John


That's actually a slightly odd set of symptoms. The fact that it is altered
by the volume control, indicates that the hum is getting in 'front end',
which is usually down to a grounding issue on the input signal cable or
whatever. However, that being the case, the hum would not normally be
expected to be present on all inputs (and presumably on both channels ?).
OTOH, a power supply fault that is causing hum, will not usually be affected
by the volume control.

I suppose it is possible that one of the electrolytics in the power supply
is getting poor - it is certainly old enough to have electro trouble - but
it is my experience that these older types rarely fail, especially if they
*have* been going for 30 odd years without trouble. It would, I think
though, still be my first move to check them either for ESR, or better, for
ripple present on them. If you don't have a 'scope, you could get an idea of
the ripple level by measuring the voltage across them, with the meter set to
an AC volts range. You should struggle to get any reading much above zero.
After that, I would be looking at a bad regulator transistor, such as that
one at the left hand side, bolted down under the transformer. I've known
rails that have gone high from a short or leaky regulator, cause such
disturbance to preamps.

Arfa